High intensity LEDs are becoming more popular as light sources for traffic lights, automobile interior and exterior lighting, signboards, and other applications. The light output of a silicon LED chip is not only a function of the chip size and the process, but also a function of the junction temperature of the LED. By keeping the junction temperature low, LEDs can be driven with twice or three times as much current and, thus, generate twice or three times the light output while still extending the life of the LEDs.
Junction temperature affects LED and laser-diode performance in many ways. Light output center wavelengths, spectrum, power magnitude, and diode reliability are all directly dependent on the junction temperature. Thus, thermal design of the diode itself and the packaging in which it is encased becomes crucial to the overall performance of the device. Validation of thermal design and assembly repeatability requires the ability to measure junction temperature.
LED performance can be accurately controlled if the temperature Tjunction at the LED junction is known. The junction temperature Tjunction of a LED can be determined based on the forward voltage Vf of the LED at a small drive current (in the order of e.g. 10 μA) using the following equation:Tjunction=TRef+(Vf—measured−Vf—Ref)/TCoeff  (1)where TRef (° C.) is the reference temperature, Vf—Ref (V) is the forward voltage of the LED at the reference temperature and at the same drive current, and TCoeff (V/° C.) is the temperature coefficient, which relates the change in LED forward voltage to the change in its junction temperature.
FIG. 2 shows a diagram illustrating the relation between the forward voltage Vf and the junction temperature Tjunction of an exemplary LED. From the linear relationship depicted in FIG. 2, it is clear that to be able to determine the junction temperature of a LED accurately, one must know the values of the reference temperature Tref, the forward voltage Vf—ref and the temperature coefficient TCoeff. These parameters can be determined beforehand for a type of LED in laboratory condition. However, this predetermination of reference values add cost, effort and complexity to LED or LED fixture manufacturers.
The U.S. Pat. No. 7,052,180 discloses an LED junction temperature tester which measures the LED junction temperature directly by taking advantage of the linear relationship between the forward current driven through the LED, the forward voltage of the LED, and the junction temperature, to determine the LED junction temperature. Calibration is conducted by placing two LEDs from the same family in ambient temperature and passing a small test current through each of the LEDs to obtain the forward voltage of the LED at ambient temperature. The LED under test is then placed in an environmentally-controlled chamber where the temperature is raised a known amount above ambient temperature. Known low and high voltage values are associated with the ambient temperature and the environmental chamber temperature, causing the LED under test to become a calibrated thermometer than can measure its own junction temperature due to the linear relationship between the forward voltage and the junction temperature.